Tenebrae

The episode "The Crystalling" seems to challenge that idea. Everyone was surprised that a pony could be born an alicorn. Rarity states that its something which has to be earned (something that even Megan McCarthy confirmed). Even Luna states that such a thing is beyond her understanding.

Actually "Horse Play" shows that Starswirl was still present after the foundation of Equestria which would mean that the age of the Pillars occurs after the Unification.

As far as we know yes. Cadance was able to after all.

Wow, that's quite the essay you've got there. Quite an interesting read too. However if you'll forgive me I have a few critiques: 1. I don't think Celestia and Luna were crowned princesses immediately after the Unification. "Horse Play" shows us that Celestia was only a student during the early days of Equestria and only came to prominence after she moved the Sun for the first time. My guess is that she became an alicorn shortly afterwards and then was crowned. 2. The Pillars were founded after the Unification and so could not have been accomplished by them. PS: I do not think the Journal of the Two Sisters is canon anymore. It has several inaccuracies in it that I no longer use it as a valide source of information.

I don't think so, otherwise Daybreaker would not have introduced herself to Celestia. Luna knowing of her was probably lazy writing.

I don't see how Celestia could kill Discord anyways as she is far weaker than him. Even Celestia and Luna combined cannot defeat discord. Only the Elements of Harmony or something similar to the Black Stone Throne can neutralise him.

As far as we can tell, yes Celestia can move the sun whenever she wants and it doesn't seem difficult as its her special talent. As for the rest I am not sure. Equestria is fictional fantasy setting about a show targeted to little girls. Expecting it do work the same way our world does is a bit naïve. We already know that day and night in Equestria is not the same thing as it is in our world as seen when day and night can occur simultaneously and the fact that the sun can be out during nightime. The only evidence that Equestria is round is a globe found in Twilights room but for all we know its not to be taken seriously and perhaps Equestria is flat instead. Its also highly likely that the Moon and the Sun are not as big and not as far away as they are in our world. It's enough to give you an aneurysm I tell you.

It is most likely lazy designing. Remember this is a show who's target audience is supposed to be young girls so a certain level of artistic inaccuracy is to be expected. It could also be that Equestria is located on the other side of the globe. Another idea is that the globe is not meant to be taken seriously and Equestria is actually flat. Until a pony goes into space we won't know for sure. Besides the world of MLP is a strange place where day and night can happen simultaneously and where the sun can be out at night.

There is no reason for her to return. Celestia never gave in to darkness and turned into Daybreaker before so there is no reason to believe that she will especially considering G4 is almost over. Also Celestia saying "you will never be real again" is a big clue-in regarding future appearances of Daybreaker. At least that's what I think.